Palladium complexes with a new hemilabile bis(oxazoline)phenylphosphonite ligand. Characterization of an unprecedented chloro palladium(II)-(eta(1)-allyl) complex

The coordination chemistry of the new, structurally characterized ligand bi s(oxazoline)phenylphosphonite (I, abbreviated NOPONMe2), shows its flexibil ity which is due to the possible formation of six-membered chelate rings. I n the Pd(II) complexes [Pd(NCMe)-(NOPONMe2-N,P,N)] (BF4)(2), 1 (characteriz ed by X-ray diffraction in 1.0.5Et(2)O.0.33MeCN), and [PdCl(NOPONMe2-N,P,N) ] (PF6), 2, this ligand behaves in a static tridentate manner, whereas in [ Pd(Me)Cl(NOPONMe2-N,P)],3, [PdI2(NOPONMe2-N,P)],4, [PdCl2(NOPONMe2-N,P)], 5 , and the allyl complex [Pd(eta (3)-C3H5)(NOPONMe2-N,P)](PF6), 6, it displa ys fluxional bidentate behavior, as shown by variable-temperature NMR studi es. In 3, only the isomer in which the methyl ligand is trans to nitrogen i s formed. In the related complex [Pd(eta (3)-C3H5)(NOPONMe2-N,P)]Cl, 7, an equilibrium has been evidenced between 7a and 7b, which involves coordinati on of the chloride and isomerization of the allyl ligand from eta (3) to et a (1). The latter isomer is quantitatively formed in toluene at 259 K and i n the solid state. This was established using NMR spectroscopy by combined variable-temperature solution and solid-state studies. Isomer 7b was also c haracterized by X-ray diffraction, a rare example of a fully characterized allyl yl-bonding mode for Pd complexes and the first in transition metal ch emistry for a mutual cis arrangement of eta (1)-allyl and chloride ligands, a situation relevant to intermediates involved in catalytic transformation s. The tridentate coordination mode of I found in complexes 1 or 2 never oc curred in the related alkyl or allyl complexes. This is consistent with the antisymbiotic effect between carbon and phosphorus donors, and this findin g was confirmed by theoretical calculations. To understand whether the mutu ally cis disposition in 3 and 7b of the chloride ligand (trans to P) and of a sigma -donor ligand such as the methyl or the eta (1)-allyl ligand (tran s to N) is intrinsic to the nature of these ligands or related in one way o r another to the P,N heterobidentate nature and resulting asymmetry of the NOPONMe2 ligand, DFT-B3LYP calculations were carried out on a series of iso meric structures of four- and three-coordinate chloro, methyl, and eta (1)- allyl Pd(II) complexes. The existence of an energetic barrier against the f ormation of a compound where the phosphorus atom of tridentate NOPONMe2 is trans to an alkyl or eta (1)-allyl ligand was established.